1. Technical Field
The invention relates to a variable power optical system for use in a video camera and an electronic still camera and particularly, relates to a compact zoom lens suitable for a surveillance dome camera used from a visible range to a near-infrared range, and the like.
2. Description of the Related Art
As a power variable optical system for use in a video camera or an electronic still camera, such a two-group type zoom lens has been known that power variation is executed by moving a second group along an optical axis and correction of an image plane associated with the power variation is made by moving the first group. In particular, for the purpose of a surveillance camera, it is required of such a zoom lens system that good chromatic aberration should be maintained. Japanese Patent No. 3600870 and JP 2004-317901 A (corresponding to U.S. Pat. No. 7,046,454) disclose a lens configuration that chromatic aberration is corrected in a surveillance camera lens system including a negative first group and a positive second group. More specifically, Japanese Patent No. 3600870 discloses such a four-group/five-lens configuration that two positive single lenses are arranged on the object side in the second group and a cemented lens and one positive single lens are arranged subsequently. This cemented lens has a negative refractive power as a whole, and is formed of a negative lens and a positive lens in order from the object side. Also, JP 2004-317901 A discloses such a three-group/four-lens configuration that one positive single lens, a cemented lens formed of a positive lens and a negative lens, and one positive single lens are arranged in order from the object side in the second group.
However, in the optical system for the surveillance camera, nowadays high variable power as well as correction of the chromatic aberration is required. In the above configurations disclosed in Japanese Patent No. 3600870 and JP 2004-317901 A, a variable power ratio is about two. However, variable power (e.g., three or more) higher than this ratio is desired. Meanwhile, with spread of the compact surveillance dome camera, such a demand that an optical system is made so small as to be fitted in the dome is increasing. Also, in the use of the surveillance camera, it is preferable that the chromatic aberration should be corrected satisfactorily from a visible range to a near-infrared range in each variable power range. In the lens disclosed in Japanese Patent No. 3600870, the cemented lens having the negative power as a whole in which the negative lens is arranged in the front portion is used in the second group in order to correct the chromatic aberration. However, with this configuration, the cemented lens is increased in size. For this reason, in order to satisfy higher power variation, this configuration is disadvantageous in size reduction and aberration correction. Therefore, a compact and high variable power zoom lens whose chromatic aberration is satisfactorily corrected particularly from the visible range to the near-infrared range is demanded.